Recently, many nanomaterials, such as graphene, carbon nanotubes (CNTs) 21 and nanoparticles (NPs), 22 have been developed and applied into electrochemical areas. 20 Nevertheless, the sensitivity and reproducibility usually tend to be the issue because of interference from other biological molecules like dopamine (DA) and uric acid (UA), leading to the challenge of employment in food, drug or real sample analysis.
19 With modified electrode surface, the electrochemical response is promoted enormously, and the lower limit of detection (LOD) and wider linear range are accessed. 18 Beyond these, electrochemical techniques are often applied, which are easy to implement and not expensive. Nowadays, diverse methods have been developed to improve the detection of AA, including ultra- and high-performance liquid chromatography (UPLC or HPLC), 15 capillary electrophoresis, 16 fluorescence spectroscopy 17 and UV-Vis spectroscopy. This is exactly significant not only for monitoring human metabolism, but also for the supervision of food, drugs and dietary supplement. 11–13 Additionally, because AA is in a millimole or even smaller scale, particularly in human bodies, novel facile and rapid methods contributing to selective and sensitive detection are required. 10 Therefore, the determination of AA concentration is of great use, which could be considered as an important physiological indicator for anti-aging. It was reported that an AA shortage could lead to the symptoms of scurvy, 9 however, exaggerated amounts could induce stomach convulsions.
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5 Recently, due to the crucial functions in free radical scavenging, 2–4 cell development and therapeutic areas, such as wound healing, preventing cancer and enhancing immunity, 8 AA has continuously attracted the public's interest. 6 It is also an important water-soluble vitamin, which exists extensively as a highly active species participating in the metabolic processes of many creatures, 7 fresh fruit and vegetables. 1 It is capable of being reduced, 2,3 and thus is widely used as a natural antioxidant in food, 4 juice, 5 medicine 3 and cosmetics. Introduction Ascorbic acid (AA), which is also known as vitamin C, is a polyhydroxy compound with a similar structure to glucose.
It was also successfully applied to the real sample testing of various AA-containing tablets. Moreover, the novel AA sensor demonstrated good reproducibility, favourable stability and high sensitivity towards glucose, uric acid (UA), dopamine (DA) and several amino acids. The superb response could be ascribed to the porous nanosheet structure of HKUST-1, which enhanced both the effective surface area and the electron transfer ability significantly. The limit of detection (LOD) was 3 μM at S/N of 3. Under optimal conditions, the oxidation peak current at +0.02 V displayed a linear relationship with the concentration of AA within the ranges of 0.01–25 and 25–265 mM, respectively. An equal-electron-equal-proton reaction was deduced from the pH investigation, and a diffusion-controlled process was reinforced by the dynamics study. This material was then loaded onto the surface of an indium tin oxide (ITO) electrode to catalyse the electrochemical oxidation of ascorbic acid (AA). Its morphology, structure and composition were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman spectroscopy, nitrogen adsorption and desorption isotherms, energy dispersive X-ray spectroscopy (EDS) and elemental analysis (EA). In this work, a Cu-based nanosheet metal–organic framework (MOF), HKUST-1, was synthesised using a solvent method at room temperature.
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